There is a growing demand for secondary batteries necessary in mobile electronic devices for information communication such as PDA, mobile phones and laptop computers, or electric bike and electric vehicle, and in keeping up with the trend towards compact and lightweight electronic devices, efforts are being made to commercialize secondary batteries that reduce in size and weight, has high energy density, and can be charged and discharged with high capacity.
Secondary batteries in typical shape include, for example, prismatic or pouch-type lithium ion secondary batteries with a small thickness for use in electronic devices such as mobile phones. However, prismatic or pouch-type secondary batteries which are in flat shape fail to efficiently use an internal space, and cannot be adapted for secondary battery applications in various shapes and sizes. Therefore, there is the demand for battery cells and battery packs having a shape that efficiently uses an internal space and has high capacity for applications of electronic devices such as mobile phones in streamlined shape and/or size.
Conventional methods for fabricating a curved battery include a method by which a flat plate-shaped electrode assembly or battery is produced and pressed by a curved jig to fabricate a curved battery.
In relation to this, there are some earlier technologies that form a curve on an electrode assembly. For example, US Patent Publication No. 2007/0059595 discloses a battery in which a jelly-roll type electrode assembly has a curved cross section perpendicular to a winding axis. According to the technology, it is disclosed that the curved shape is achieved through thermal compression molding using a concave heater and a convex heater.
Furthermore, Korean Patent Publication No. 2014-0104888 discloses a method by which a flat battery cell containing an injected electrolyte solution is mounted between a pair of curved jigs and pressed to fabricate a curved battery.
Generally, if a plurality of plate-shaped stack structures is bent to a predetermined of curvature radius together, an amount of deformation in an inner layer stack structure is larger than that of an outer layer stack structure. In contrast, due to the repeated expansion and contraction of active materials coated on a current collector during charge and discharge, a secondary battery cannot maintain its transformed state in a predetermined shape. Thus, a secondary battery with a structure in which a curvature radius is small, namely, a secondary battery with a structure in which warpage is relatively large may tend to restore the warped state to its original state during repeated charge and discharge, and in this case, current collectors are pressed at their ends by a cell case and are subjected to a strong force enough to penetrate a separator, causing a short.
When stress concentrates on a central part of a cell, deformation such as a warpage may occur, and particularly, as the curvature radius reduces, this tendency increases, so the range of shapes that can be fabricated is extremely limitative. In addition, because an electrode plate shrinks and expands during continuous charging and discharging of a battery, there is concern about accelerated shape changes of an electrode assembly. Therefore, there is a need to newly develop a method for manufacturing a curved battery with outstanding shape stability.